The present invention relates to the machining of threads and, more particularly, to the machining of threads by thread whirling.
Thread whirling is a cutting process performed on a lathe, milling machine, or dedicated thread whirling machine, in which a thread is produced on the outside diameter of metallic or non-metallic material (work-piece). The shape to be imposed is produced by cutting elements (tools) mounted within a hollow milling head (tool holder ring), which rotates around the outside diameter of the work-piece. The work-piece may be rotated in the same direction as the milling head and at a relatively lower speed. The axis of rotation of the work-piece is offset from the axis of rotation of the milling head, and the axis of rotation of the milling head is tilted at the desired helix angle of the thread. The work-piece is fed along its axis of rotation through the milling head, or the milling head moves along the axis of rotation of the work-piece. The work-piece is fed, or the milling head is moved, at the desired lead of the thread (i.e., the desired distance from the root of one thread to the root of the next thread). The cutting elements have cutting surfaces that successively and tangentially contact the work-piece as the milling head orbits around the work piece.
With thread whirling, screws can be machined as much as four times faster than other thread methods, with a much superior surface finish. Additionally, in comparison to a single point thread cutting method, there is no pre-shaping of the material required, the length of the part to be cut is not limited, and tool life is substantially increased.
One example of a thread whirling machine is provided in U.S. Pat. No. 4,278,374. In this whirling machine, a plurality of cutting elements is disposed in a milling head (tool holder ring). Adjustments in the relative positions of the cutting elements with respect to the tool holder ring and work-piece are carried out by bolts on the cutting element. The heads of the bolts bear against an inner wall of the cavities in the tool holder ring in which the cutting elements are disposed and the bolts are extended or retracted to adjust the position of the cutting elements.
Another example of a thread whirling milling head is shown in FIG. 1. The milling head 10 of FIG. 1 has a face 11, onto which three circular cutting elements 12 are secured. Each of the circular cutting elements 12 include a cutting surface 14 formed thereon, which extends radially from the center of the circular cutting element 12. The cutting surfaces 14 contact a work-piece 16 as the work-piece 16 and milling head 10 rotate, forming threads into the outside diameter of the work-piece 16. If the cutting surfaces 14 become dull, they are reground to form new cutting surfaces 14′, and the work-piece 16 is rotated such that the new cutting surfaces 14′ are in a proper cutting position (e.g., in the proper cutting height and rake angle) relative to the milling head 10. The circular shape of the cutting elements 12 provides for good tool strength and makes regrinding easy. However, setting the circular cutting elements 12 in proper cutting position in the milling head 10 is a difficult and a time consuming process that requires special tools such as a setting gauge, a setting jig, and a knock out device.